In a traditional design boilers/furnace you run hot combustion gases through a heat exchanger (HX) to heat water/air and then into a flue. The problem is that i

In a traditional design boilers/furnace you run hot combustion gases through a heat exchanger (HX) to heat water/air and then into a flue. The problem is that if it cools too much in the HX it can condense in the flue. With the condensate being extremely corrosive it will quickly destroy the flue and can cause all kind of issues from CO poisoning to fires. So the HX is small enough to keep the gases hot in the flue to prevent corrosion. A more complicate design has a big HX and various pumps and fans to drain the condensate and push out the exhaust gases. It uses a PVC flue and drain line to withstand corrosion but at the end you still have moving parts exposed to the acidic condensate and it does not have the service life of a straight metal flue design. That, combined with the higher cost of a condensing heater (because of all the additional pumps and fans and corrosion protection), calls for a careful cost analysis since you need to burn a lot of gas to justify quicker replacement and more service calls for a more expensive unit.